add calculated_remotes (#759)

* add calculated_remotes

This setting allows us to "guess" what the remote might be for a host
while we wait for the lighthouse response. For networks that hard
designed with in mind, it can help speed up handshake performance, as well as
improve resiliency in the case that all lighthouses are down.

Example:

    lighthouse:
      # ...

      calculated_remotes:
        # For any Nebula IPs in 10.0.10.0/24, this will apply the mask and add
        # the calculated IP as an initial remote (while we wait for the response
        # from the lighthouse). Both CIDRs must have the same mask size.
        # For example, Nebula IP 10.0.10.123 will have a calculated remote of
        # 192.168.1.123

        10.0.10.0/24:
          - mask: 192.168.1.0/24
            port: 4242

* figure out what is up with this test

* add test

* better logic for sending handshakes

Keep track of the last light of hosts we sent handshakes to. Only log
handshake sent messages if the list has changed.

Remove the test Test_NewHandshakeManagerTrigger because it is faulty and
makes no sense. It relys on the fact that no handshake packets actually
get sent, but with these changes we would send packets now (which it
should!)

* use atomic.Pointer

* cleanup to make it clearer

* fix typo in example

udp/udp_all.go

@@ -64,6 +64,22 @@ func (ua *Addr) Copy() *Addr {
 	return &nu
 }
 
+type AddrSlice []*Addr
+
+func (a AddrSlice) Equal(b AddrSlice) bool {
+	if len(a) != len(b) {
+		return false
+	}
+
+	for i := range a {
+		if !a[i].Equals(b[i]) {
+			return false
+		}
+	}
+
+	return true
+}
+
 func ParseIPAndPort(s string) (net.IP, uint16, error) {
 	rIp, sPort, err := net.SplitHostPort(s)
 	if err != nil {